The count of 260 comes from the product of the number of letters, A-Z, which is 26, and the number a single-digit numbers, 0-9, which is 10. There are 260 license plates of this two-digit form. (The first digit is the letter, the second is the number.)

If both digits can be letter or number, then there are 36 possibilities for each digit. The number of license plates of this form is 36*36 = 1296.

Wikipedia gives an overview of the letter / number combinations allowed on different countries’ license plates, whose forms generally involve many more digits than the simple examples analyzed here.